The present embodiments relate to electronic devices and are more particularly directed to a single-antenna interference cancellation receiver for use by way of example in a global system mobile (“GSM”) communication system.
Wireless communications are very prevalent in business, personal, and other applications, and as a result the technology for such communications continues to advance in various areas. After cellular communication systems originated in the United States, one type of cellular system that then followed in Europe, and which is now finding its way into the United States as well as elsewhere, is the GSM system. By way of introduction, the following discusses certain aspects of GSM, while numerous other aspects will be known in the art. As its name suggests, GSM has become globally accepted and it provides a set of recommendations so that uniform concurrence with them permits compatible communication between different equipment in numerous geographic regions. As a cellular system, GSM is characterized by providing low-power base transceiver stations (“BTSs”, or singular, “BTS”). Each such BTS communicates signals with mobile units that are within a geographic area, or “cell,” reachable via wireless radio signal communications with that BTS. A single BTS may have a number of actual transceivers, typically based on the expected density of users in the cell corresponding to that BTS. A group of BTSs is often controlled by a common base station controller. The controller typically provides all the control functions and physical links between the BTS and a mobile services switching center, where the controller is a high-capacity switch that provides functions such as handover, cell configuration, and control of radio frequency power levels in BTSs.
For GSM communications, both control and traffic (i.e., speech and data) channels are digital, and GSM uses a combination of time division multiple access (“TDMA”) and frequency division multiple access (“FDMA”). Particularly, for the FDMA aspect, the 25 MHz band for the link, either uplink or downlink, is divided into 124 carrier frequencies (separated by 200 kHz) and one or more of these carrier frequencies is assigned to each BTS with some level of frequency hopping. The BTS then divides each of its carrier frequencies into time slots, thereby providing the TDMA aspect. The fundamental unit of time in this TDMA scheme is called a burst or a burst period, which lasts 15/26 ms (or approximately 0.577 ms). One physical channel is one burst period per TDMA frame. Channels are defined by the number and position of their corresponding burst periods. Eight burst periods are grouped into a TDMA frame (120/26 ms, or approx. 4.615 ms), which forms the basic unit for the definition of logical channels. Further, a group of 26 TDMA frames forms a 120 ms multiframe, and that 120 ms is how the length of a burst period is defined, namely, by dividing the 120 ms by 26 frames and further dividing that result by 8 burst periods per frame. The multiframe provides a traffic channel (“TCH”) that carries speech and data traffic. TCHs for the uplink and downlink are separated in time by three burst periods, so that the mobile station does not have to transmit and receive simultaneously, thus simplifying the mobile station electronics.
Mobile stations in GSM take various forms, but as introduction to the preferred embodiments described later, the present discussion focuses on single-antenna mobile units. In such a unit, circuitry receives signals from the BTS of the cell in which the mobile station is located and decodes the signals into corresponding data. In this regard, however, the mobile station also will concurrently receive interference, such as from BTSs in other cells (or, with respect to the BTS of the cell in which the mobile station is located, from that BTS's antenna's transmissions to the two 120 degree sectors in which the mobile station is not located). For purposes of assisting with signal decoding such as in implementing single-antenna interference cancellation (“SAIC”), the GSM burst includes a known sequence of data referred to as a training sequence code (“TSC”). Generally, when a mobile station receives a signal, an estimable correlation of interference may be made from the difference between the samples of the channel-corrected received TSC signal and the known TSC, where this estimate may be found by way of example as the square root of the inverse of the correlation matrix of these samples. Thus, this estimation is then used to achieve interference cancellation in the entirety of the signal, thereby permitting a certain level of performance for decoding all of the data in each burst.
While the preceding approach has proven workable in various implementations, the present inventors have recognized that the existing performance in interference cancellation and the resulting data decoding may be improved. Indeed, recently SAIC has been made quite popular in GSM standardization due to its potential in providing a significant capacity increase for high-frequency reuse GSM networks. However, such networks could be severely limited by co-channel interference. While several possible SAIC algorithms may be used, some require the interfering user information such as their presence, timing, channel estimates, and TSCs; examples of such algorithms include serial interference cancellation (“SIC”) and joint maximum-likelihood sequence estimation (“JMLSE”). However, other SAIC algorithms are sometimes referred to as “blind capable” because they do not require this information and, thus, are more attractive. Thus, it is desirable to support and improve the performance of an SAIC algorithm that does not require the interfering user information, as is achieved by the preferred embodiments described below.